ABSTRACT
The mouth anlage develops from two sides. The pharyngeal cavity gradually penetrates into the entodermal oral evagination, while the ectodermal mouth plate slowly invaginates, forming the stomodeum; the lumina of the entodermal evagination and the ectodermal invagination gradually approach each other. The visceral pouches become deeper and the corresponding furrows in the contour of the embryo change gradually. The first and second furrows flatten at stage 35/36, whereas the third gets deeper. The partition wall between pouch and furrow becomes thinner, a process which continues during the following stages. The rostrally directed rows form tooth-like protrusions which fit in between the club-shaped, caudally directed “rakes” of the preceding arches. The tooth-like protrusions on the fourth arch are the most advanced. The thyroid anlage appears as a caudally directed finger-shaped protrusion of the oro-pharyngeal floor at the level of the first visceral pouch at stage 33/34.
